This invention relates to biodegradable polyesters and to articles prepared from such polymers.
Biodegradable polyesters of both natural and synthetic origins are well known. See for example, Encyclopedia of Polymer Science and Technology, Second Edition, Volume 2, pp. 220-243. However, these known biodegradable polyesters have several limitations which render them unsuitable for many intended applications. For example, biodegradable polyesters of natural origin which are isolated as products of fermentation processes require extensive separation and purification in order to provide a product of purity suitable for typical plastic uses. These polymers also have generally poor physical properties and are difficult to fabricate into useful articles. Synthetic biodegradable polyesters are also known. Although these synthetic polyesters do not have the separation and purification problems associated with natural materials, they often suffer from the deficiencies in mechanical properties and fabricatability encountered in natural biodegradable polymers.
Synthetic thermoplastic polyesters can be prepared from the reactions of difunctional acids and alcohols. These diacids and dialcohols are generally derived from petroleum-based sources, whose prices are subject to extreme fluctuation on the world market. It would be desirable to make use of raw materials derived from renewable resources such as the products of agriculture, for the production of new plastic materials. Dicarboxylic acids derived from plants, such as tartaric acid from grapes, or malic acid from apples, could be useful for such purposes, except that these particular diacids also contain hydroxyl groups.
It is well known in the art of polyester synthesis that the use of reactants that contain more than two reactive-functional groups per molecule, as is found in the above-mentioned plant-derived acids, will lead first to branching, and then to gelation and crosslinking. Thus, preparation of thermoplastics from hydroxy-functional diacids are only possible if a suitable protecting group is employed to prevent reaction of the hydroxyl moieties during the polymerization. Deprotection of the hydroxyl groups provides a hydroxyl-functional polyester. However, this process requires the use of costly reagents, and additional undesirable separation and purification processes to provide a product suitable for use as a practical plastic material.
It would be desirable to provide synthetic thermoplastic polyesters derived from hydroxy-functional dicarboxylic acids, and to provide useful articles from such polyesters that are biodegradable.